First you have to wait for the arm to come up then hit it with the cane do this with both arms then he will shoot out blue balls of light split in to four then hit back the balls just keep repeating this to defeat him (it is a good idea to have great spin attack)

To get light arrows, go as soon as you can to the house on the west side
of Hyrule town. In a house there lives someone wich you can fuse
Kinstones with. his name is stranger. fuse a red kinstone with him to
open a portal near the house where you start. Go in to get in a house of
the wind tribe. on the second floor at the door on the most right, lies
someone on a bed. A ghost circles around his head. Suck it up using the
gust jar( you get it in level one). After the wind ruins, the man will
stand next to the inn in hyrule town. fuse kinstones and follow him to
get the light arrows.

When you go into cave where you have to use your claw things that breakthrough walls, you will randomly see these circles (with a line going up and down it) in some of the caves' walls, fuse a stone with them, and eventually youll get all the creatures.

As you know you get the Cane of Pacci...Well this is what you need.
Run behind or too the side of the boss.Use your Cane of Pacci on its shell.The shell will filp and his head will fall on land making a path from land too its shell.Go too the shell and attack it.You'll have to do this about 4 or 5 times. (after everytime you do this,the boss will breathe more fire).

A2:

We'll the fire boss os the 2sd out of 4 . We'll the item called the cane of pacci . you will need it plus the gust jar . Because He'll blast fire out of his mouth and every time you hit you'll need the sword so do fast item changes . I mean start select the idem you need and here's a tip always keep it on the item screen . So Have a fun time defeating him -Answer by:rosevictor